This invention relates to variable pitch ducted fans and particularly to an improved variable area discharge convergent-divergent nozzle of the duct and constitutes an improvement over the flap design disclosed in a companion application Ser. No. 405,605 entitled Bypass Ducted Fan Propulsor by Carl Rohrbach filed on even date and assigned to the same assignee.
While the utilization of variable flaps on a duct are well known in the art, such flaps heretofore would never be designed to form a low area ratio convergent-divergent nozzle. With the advent of the variable pitch fan aircraft propulsors which are utilized to provide reverse capabilities it has become desirable to provide bellmouth shaped inlet of the fan duct exit.
I have found that I can improve the effectivenss of the duct for both the cruise and reverse mode performances by designing the flap to provide a convergent-divergent nozzle when the flaps are in their closed position for cruise operation and when in the open position form a desirable inlet for the reverse pitch operation. Moreover, I have found that for low fan pressure ratios I can increase the flow coefficient of the fan duct nozzle in such a manner that during the periods of operation where the pressure ratio across the fan is at a low value the improvement in the flow coefficient tends to move the operation of the fan away from the surge line and enhances the effectiveness of the fan particularly during takeoff, landing and ground operation.
Additionally by disposing the pivot point of the flap in a discrete and judicious location serves not only to prevent flow disturbances through the duct but tends to improve the effectiveness of the bellmouth shaped inlet during reverse mode.